Meiosis and Sexual Life Cycles
People get their traits from their parents. Parents pass coded information in the form of
hereditary units called genes.
The thousands of genes we inherit from our parents make up our genome.
Our genes control the traits we have as we develop from embryos to adults.
Genes are segments of DNA. DNA is a polymer of four different kinds of monomers called
nucleotides. (These nucleotides are adenine, thymine, cytosine, and guanine)
Each gene has its own sequence of nucleotides. Cells translate these sequences.
Humans have 46 chromosomes, other species also have specific amounts. Chromosomes are long
pieces of DNA wound around proteins.
A single chromosome can have hundreds or thousands of genes. A gene’s specific location on a
chromosome is called the gene’s locus.
In asexual reproduction, a single organism is a parent and passes on all its genes to offspring.
Since the parent and the offspring have the same genome, the offspring are clones. There is no
variation in the genome, except if a mutation occurs (very rare!)
In sexual reproduction, there is much more genetic variation. Two parents create offspring that
have unique combinations of genes from the parents.
In a human somatic cell, there are 46 chromosomes.
The 46 chromosomes are grouped in pairs, there are two of each type of chromosome. When
these chromosomes are mapped out, they form a karyotype.
The chromosomes that make up a pair (they are the same length and have the same centromere
positioning) are called homologous chromosomes.
The two homologous chromosomes have genes that control the same inherited characters. For
example, if a gene for eye color is on one chromosome, then the homologous chromosome will
also have a gene for eye color.
There is one exception to the rule of homologous chromosomes. This is the sex chromosome
pair. Females have two X chromosomes. Males have one X and one Y chromosome.
X and Y are called sex chromosomes. The other chromosomes are called autosomes.
Each homologous chromosome is inherited from a different parent. One chromosome of the pair
is from Dad, and one chromosome from the pair is from Mom.
Gametes (sperm and ova) are different from somatic cells because they have half the number of
chromosomes. For example, human gametes have 23 chromosomes. Each gamete has 22
autosomes and 1 sex chromosome.
A cell with a single chromosome set is called haploid (n). n is the haploid number. In humans’
case, n=23.
After sex, two gametes fuse together and form a zygote. This process is called fertilization (or
syngamy). Now zygote has a set of chromosome pairs. The cells resulting from the zygote are
called diploid (2n). In our case, 2n=46.
All the genes from the zygote are in all of the cells of the organism. These cells come from the
mitosis in the zygote.
However, gametes are not formed by mitosis. Meiosis is the process that makes gametes, by
halving the number of chromosomes in an autosome.
Different Sexual Life Cycles
Although meiosis and fertilization occurs in all sexually reproducing animals, the life cycle of
the species varies.
In animals, Gametes are the only haploid cells. Two gametes fuse to form a zygote, and the
zygote reproduces by mitosis, eventually forming the organism
In protists, things are reversed. After gametes fuse to form the zygote, the zygote undergoes
meiosis. Then the haploid cells from meiosis multiply by mitosis to form a multicellular haploid
organism.
Plants and other species of algae have a type of lifestyle called alternation of generations.
-This includes both haploid and diploid multicellular stages.
-The diploid multicellular stage is called the sporophyte. Cells in the sporophyte undergo
meiosis and create spores. Spores are haploid
-A spore will multiply by mitosis and become a multicellular organism called the gametophyte
-The haploid gametophyte will produce gametes by mitosis.
-When two gametes from the gametophyte fuse, fertilization occurs and forms a diploid zygote.
The zygote multiplies by mitosis and becomes the sporophyte.
Meiosis
Meiosis is kind of like mitosis. Before meiosis, chromosomes replicate during interphase.
However, after replication of DNA, there are two cell divisions, called meiosis I and meiosis II.
Meiosis results in four haploid daughter cells.
The steps of Meiosis
Interphase I: All DNA is replicated before mitosis.
Prophase I:
Chromosomes condense and homologous chromosomes come together in a process called
synapsis. In synapsis, homologous chromosomes with two sister chromatids each come together
as pairs.
The joined chromosomes form a tetrad, because there are four sister chromatids in the pair.
At numerous places along the chromosomes, chromatids of the homologous chromosomes are
criss-crossed.
The places where they cross are called chiasmata.
Centrosomes move away from each other and the nuclear envelope disappears.
Finally, the meiotic spindle latches onto the kinetochores of chromosomes.
Metaphase I:
Tetrads are now arranged on the metaphase plate. Kinetochore microtubules attach to each
chromosome of the tetrad.
Anaphase I:
The mitotic spindle pulls homologous chromosomes of the tetrad apart and they move to
different poles of the cell.
However, sister chromatids stay connected at the centromere.
Telophase I:
Homologous chromosomes continue to move across the cell to the poles of the cell.
Now each pole has a haploid chromosome set, but each chromosome has two sister chromatids.
Two haploid daughter cells are formed.
Prophase II:
The spindle apparatus forms yet again, and the chromosomes move towards the metaphase II
plate.
Metaphase II:
The chromosomes are positioned on the metaphase plate. (like mitosis)
Anaphase II:
The meiotic spindle seperates sister chromatids and pull them to opposite poles of the cell.
Telophase II and Cytokinesis:
Nuclei form and cytokinesis occurs. Now there are four haploid cells.
Origin of Genetic Variation
Three things contribute to genetic variation in sexual reproduction.
1. Independent Assortment of Chromosomes.
-There are different ways that chromosomes will be distributed into haploid cells.
-The way that chromosomes line up on the metaphase I plate is how chromosomes are
assorted.
The more chromosomes, the more combinations possible for gametes.
The equation to find the number of possible combinations is 2 ^n, where n is the haploid
number of the organism.
2. Crossing-Over
-The process crossing over produces chromosomes that have genes from both mom and
dad. The chromosomes that genes are nonsister chromatids.
This occurs during prophase I.
When homologous chromosomes first come together during prophase I, they are paired
very precisely.
Chromosomes exchange portions of DNA along spots called chiasmata.
3. Random Fertilization. A female’s gamete has many possible combinations, and a male’s
gamete also has many possible combinations. So a zygote made from these two is also
very random!